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CHAPTER

4EXTERNALGROUNDING(EARTHING)

This chapter provides requirements and guidelines for designing and installing the external

grounding (earthing) electrode system at a communications site.

This chapter provides information on the following topics:

Lightning Activity and Exposure on page 4-3

Common Grounding (Earthing) on page 4-5

Grounding (Earthing) Electrode System Component and Installation Requirements on

page 4-7

Dissimilar Metals and Corrosion Control on page 4-34

Bonding to the External Grounding (Earthing) Electrode System on page 4-40

Minimum Site Grounding (Earthing) Requirements on page 4-44

Grounding (Earthing) Roof-Mounted Antenna Masts and Metal Support Structures onpage 4-74

Grounding (Earthing) Rooftop Mounted Tower Structures on page 4-79

Special Grounding (Earthing) Applications on page 4-81

Special Grounding (Earthing) Situations on page 4-88

NOTE: Throughout this chapter the terms groundingand earthingare used synonymously.

4.1 INTRODUCTION

The requirements and guidelines in this chapter are derived from a compilation of local and

national codes, widely accepted industry codes and standards, and good engineering practices.

Such codes and standards are from, but not limited to, the following standards organizations:

Alliance for Telecommunications Industry Solutions (ATIS)

American National Standards Institute (ANSI)

Australian Standards(AS)

British Standards Institution (BS)

International Association of Electrical Inspectors (IAEI)

International Electrotechnical Commission (IEC)

Institute of Electrical and Electronics Engineers (IEEE) National Fire Protection Association (NFPA)

Telecommunications Industry Association (TIA)

Underwriters Laboratories (UL)

United States Department of Defence (DoD)

United States Federal Aviation Administration (FAA)

United States National Weather Service

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INTRODUCTION CHAPTER4: EXTERNALGROUNDING(EARTHING)

References to the specific industry codes and standards on which this chapter is based are provided

throughout. The requirements and guidelines in this chapter are provided to enhance personnel safety

and equipment reliability.

Safety of personnel and protection of sensitive electronic equipment from ground faults, lightning,

ground potential rise, electrical surges, and power quality anomalies is of utmost importance at any

communications site. Though unexpected electrical events like lightning strikes and power surges

cannot be prevented, this chapter provides design and installation information on communications site

grounding electrode systems that may help minimize damage caused by these events.

CAUTION

Grounding (earthing) and bonding alone are not enough to adequately protect a

communications site. Transient voltage surge suppression (TVSS) techniques, usingappropriate surge protection devices (SPD), shall be incorporated at a communications

site in order to provide an adequate level of protection. See Chapter 7, SurgeProtective Devices, for details and requirements.

A grounding electrode system shallhave low electrical impedance, with conductors large enough to

withstand high fault currents. The lower the grounding electrode system impedance, the more

effectively the grounding electrode system can dissipate high-energy impulses into the earth.

WARNING

The AC power system ground shall be sized appropriately for the electrical service and

shall be approved by the local authority having jurisdiction.

All site development and equipment installation work shallcomply with all applicable codes in use by

the authority having jurisdiction. Grounding systems shallbe installed in a neat and workmanlike

manner (NFPA 70-2005, Article 110.12 and NFPA 780-2004, section 1.4). Where conflicting, the more

stringent standard should be followed. Government and local codes shalltake precedence over the

requirements of this manual.

Unusual site conditions may require additional effort to achieve an effectively bonded and grounded

(earthed) site. See Special Grounding (Earthing) Situations on page 4-88in these instances.

Consultation with Motorola Engineering or an engineering firm specializing in grounding electrode

system design is recommended.

Some of the benefits of a properly designed and installed low-impedance grounding electrode system

are described below (See ANSI T1.333-2001, section 4; ANSI T1.334-2002, section 5.1; BS

7430:1998; IEC 60364-1; IEEE STD 142-1991, section 1.3; IEEE STD 1100-1999, section 3.3.1; and

NFPA 70-2005, Article 250.4 for additional information): To help limit the voltage caused by accidental contact of the site AC supply conductors with

conductors of higher voltage.

To help dissipate electrical surges and faults, to minimize the chances of injury from grounding

system potential differences.

To help limit the voltages caused by lightning.

To help maintain a low potential difference between exposed metallic objects.

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STANDARDS ANDGUIDELINES FORCOMMUNICATIONSITES LIGHTNINGACTIVITY ANDEXPOSURE

To stabilize the AC voltage relative to the earth under normal conditions.

To contribute to reliable equipment operation.

To provide a common signal reference ground.

4.2 LIGHTNINGACTIVITY ANDEXPOSURECommunications facilities shall be defined as exposed to lightning unless thunderstorm activity in the

area is an average of five thunderstorm-days per year or fewer and soil resistivity at the site is less than

10,000 ohm-centimeters (-cm). The soil resistivity shallbe measured as described in ANSI/IEEE STD

81. (ANSI T1.313-2003, section 5.1.1) See Appendix B for soil resistivity measurement methods.

Figure 4-1and Figure 4-2are maps representing typical lightning activity throughout the world. These

figures are for general informational and educational purposes only and are not indicative of current or

future lightning activity. The average amount of lightning that occurs in any given area varies

significantly from year to year.

ASIANORTH

AMERICA

SOUTH

AMERICA

AFRICA

EUROPE

AUSTRALIA

0 12 45 910 1920 3940 5960 7980 99100 139140 200+

FIGURE 4-1 LIGHTNINGACTIVITY, THUNDERSTORMDAYSPERYEAR

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LIGHTNINGACTIVITY ANDEXPOSURE CHAPTER4: EXTERNALGROUNDING(EARTHING)

FIGURE 4-2 LIGHTNINGACTIVITY, FLASHDENSITY(FLASHES PERSQUAREKILOMETER PERYEAR)

Table 4-1provides a relationship between thunderstorm days per year and lightning flashes per square

kilometer per year (BS 6651:1999, table 6).

TABLE 4-1 RELATIONSHIPBETWEENTHUNDERSTORMDAYS PERYEAR ANDLIGHTNINGFLASHESPERSQUAREKILOMETER PERYEAR

Thunderstorm

days per year

Flashes per square kilometer

per year

Flashes per square mile per

year

Mean Limits Mean Limits

5 0.2 0.1 to 0.5 0.5 0.25 to 1.35

10 0.5 0.15 to 1 1.29 0.38 to 2.59

20 1.1 0.3 to 3 2.84 0.77 to 7.77

30 1.9 0.6 to 5 4.92 1.55 to 13

40 2.8 0.8 to 8 7.25 2 to 20.7

50 3.7 1.2 to 10 9.58 3.1 to 25.9

60 4.7 1.8 to 12 12.17 4.66 to 31

80 6.9 3 to 17 17.87 7.8 to 44

100 9.2 4 to 20 23.82 10.36 to 51.8

NOTE: Information obtained from BS 6651:1999, Table 6.

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STANDARDS ANDGUIDELINES FORCOMMUNICATIONSITES COMMONGROUNDING(EARTHING

Communications facilities located at elevations significantly above the average elevation of the

surrounding terrain (such as hilltops, fire towers, airport control towers, and high-rise buildings) shall

be considered exposed to lightning regardless of thunderstorm activity and soil resistivity. (ANSI

T1.313-2003, section 5.1.1.)

Communications facilities with a tower shallbe considered as exposed, regardless of thunderstorm

activity and soil resistivity. By their very construction, radio antennas/towers are considered exposed to

the possible damaging effects of lightning. Tall structures, such as towers, buildings and antenna masts,

provide a favorable discharge point for lightning strokes. (ANSI T1.313-2003, section 5.2.3.)

Some communications facilities may be classified as unexposed if the building and tower are within the

zone of protection of a higher structure. Only a qualified engineer should determine if the

communications facility is unexposed. The following standards can be used by the engineer to help

determine if the communications facility is unexposed: BS 6651:1999, IEC 61024-1-2, NFPA 780

2004, or other applicable standard in effect and recognized by the local authority having jurisdiction.

4.3 COMMONGROUNDING(EARTHING)

At a communications site, there shallbe only one grounding (earthing) electrode system. For example,

the AC power system ground, communications tower ground, lightning protection system ground,

telephone system ground, exposed structural building steel, underground metallic piping that enters the

facility, and any other existing grounding electrode system shallbe bonded together to form a single

grounding electrode system (ANSI T1.313-2003; ANSI T1.333-2001; ANSI T1.334-2002; IEC 61024

1-2, section 2.4.4; IEEE STD 1100-1999; NFPA 70-2005, Articles 250.58, 250.104, 250.106, 800.100,

810.21, and 820.100; and NFPA 780-2004, Section 4.14).

All grounding media in or on a structure shallbe interconnected to provide a common ground potential.

This shallinclude, but is not limited to, the AC power system ground, communications tower ground,

lightning protection system ground, telephone system ground, exposed structural building steel, and

underground metallic piping systems. Underground metallic piping systems typically include water

service, well castings located within 7.6 m (25 ft.) of the structure, gas piping, underground conduits,underground liquefied petroleum gas piping systems, and so on. Interconnection to a gas line shallbe

made on the customer's side of the meter (NFPA 780-2004, Section 4.14.1.3).

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COMMONGROUNDING(EARTHING) CHAPTER4: EXTERNALGROUNDING(EARTHING)

TO SUPPLY

TRANSFORMER

SERVICE

EQUIPMENT

BUILDING

STEEL

GROUNDED

CONDUCTOR

GROUND ROD

CONCRETE

ENCASED

ELECTRODE

METAL

WATER

PIPE

GROUND RING

TO LIGHTNING

PROTECTION SYSTEM

FIGURE 4-3 COMMONGROUNDINGEXAMPLE

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STANDARDS ANDGUIDELINES FORCOMMUNICATIONSITES GROUNDING(EARTHING) ELECTRODESYSTEMCOMPONENT ANDINSTALLATION

REQUIREMENTS

4.4 GROUNDING(EARTHING) ELECTRODESYSTEMCOMPONENTANDINSTALLATIONREQUIREMENTS

WARNING

To prevent accidental damage to underground utilities, always have the local utilitycompany or utility locator service locate the underground utilities before excavating or

digging at a site.

The external grounding (earthing) electrode system may consist of, but is not limited to, the following

components, shown in Figure 4-4:

Ground rods or other grounding electrodes

Concrete encased electrode

Building or shelter ground ring

Tower ground ring

Grounding conductors

Radial grounding conductors

Guy wire grounding conductors (guyed towers only)

Tower ground bus bar

External ground bus bar

Fence grounding conductors

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A: Grounding Radials

B. Tower Ground Bus Bar and Down Conductor

C. Generator Grounding Conductor

D. Buried Fuel Tank Grounding Conductor

E. External Ground Bus Bar

F. Shelter Ground Ring

G. Fence Grounding Conductor

H. Ground Ring Bonding Conductors (2 minimum)I. Tower Ground Ring

J. Earthing Electrodes (Ground Rods)

GROUNDING(EARTHING) ELECTRODESYSTEMCOMPONENT ANDINSTALLATIONREQUIREMENTS CHAPTER4: EXTERNALGROUNDING(EARTHING)

FIGURE 4-4 TYPICALEXTERNALGROUNDINGELECTRODESYSTEM

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REQUIREMENTS

4.4.1 GROUNDING(EARTHING) ELECTRODES

Grounding (earthing) electrodes are the conducting elements used to connect electrical systems and/or

equipment to the earth. The grounding electrodes are placed into the earth to maintain electrical

equipment at the potential of the earth. Grounding electrodes may be ground rods, metal plates, concrete

encased electrodes, ground rings, electrolytic ground rods, the metal frame of building or structure, and

metal underground water pipes (NFPA 70-2005, Article 250 (III)).

NOTE: Metallic underground gas piping shall notbe used as a grounding electrode (NFPA 70-2005, Article250.52), but shallbe bonded upstream from the equipment shutoff valve to the grounding electrode

system as required by NFPA 70-2005, Article 250.104 and NFPA 780-2004, section 4.14.1.3.

4.4.1.1 GROUNDING(EARTHING) ELECTRODERESISTANCECHARACTERISTICS ANDSPHERE OFINFLUENCE

Around a grounding (earthing) electrode, such as a driven ground rod, the resistance of the soil is the

sum of the series resistances of virtual concentric shells of earth, located progressively outward from the

rod. The shell nearest the ground rod has the smallest circumferential area, or cross section, so it has the

highest resistance. Successive outward shells have progressively larger areas, therefore, progressively

lower resistances. (IEEE STD 142-1991, section 4.11 and MIL-HDBK-419A).

Rod Length

Concentric Shells

2X Rod Length

FIGURE 4-5 GROUNDINGELECTRODESPHERE OFINFLUENCE

The effect of the concentric shells is that it takes a finite amount of earth for a ground rod to fully realize

its resistance value. This finite amount of earth is commonly known as the ground rod's sphere of

influence. The sphere of influence for a ground rod is commonly thought of to be a radius around the

ground rod equal to its length; the ground rod achieves approximately 94% of its resistance value at this

radius (100% is achieved at approximately 2.5 times the rod length) (IEEE STD 142-1991, section 4.1).

See Figure 4-5.

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Optimum Spacing2X Rod Length

2X Rod Length

FIGURE 4-6 MINIMUMGROUNDINGELECTRODESPACING FORMAXIMUMEFFECTIVITY

Table 4-2provides the relationship between percentage of total ground rod resistance and the radial

distance from the ground rod (IEEE STD 142-1991, Table 9).

.

TABLE 4-2 TOTALGROUNDRODRESISTANCEVS. DISTANCEFROMGROUNDROD

Distance from Electrode Surface (r)**

Approximate Percentage of

ft Meters Total Resistance

0.1 0.03 25%

0.2 0.06 38%

0.3 0.09 46%

0.5 0.15 52%

1.0 0.3 68%

5.0 1.5 86%

10.0* 3.0* 94%

15.0 4.6 97%

20.0 6.1 99%

25.0 7.6 100%

* 94% of the resistance to remote earth occurs within a radius equal to the length of the ground rod. This

radius is commonly used as the ground rod's sphere of influence.

** Ground rod resistance at a radius (r) from a 3 m 16 mm (10 ft. 0.625 in.) ground rod(From IEEE STD 142-1991, Table 9)

The following observations can be made from the above table (IEEE STD 142-1991, chapter 4):

Within the first 2.5 cm (1 in.) from the ground rod, 25% of the total resistance to earth is achieved.

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REQUIREMENTS

Within the first 152 mm (6 in.) from the ground rod, 52% of the total resistance to earth is achieved

The area immediately around a ground rod is the most important for reducing its resistance to earth.

See Grounding (Earthing) Electrode Encasement Materials on page 4-27for information on

reducing resistance.

In high resistivity soil areas, decreasing the soil resistance in this area is useful in improving

the effectiveness of the grounding electrode system.

In porous soil areas, decreasing the contact resistance with the ground rod in this area is useful

in improving the effectiveness of the grounding electrode system.

Unless specified elsewhere in this chapter, ground rods should be installed apart from one another by

the sum of their respective lengths, so their spheres of influence do not overlap (See Figure 4-6). This is

especially important when only a small number of ground rods are installed, such as around tower

ground rings.

24.4 m (80 ft.)straight line will achieve a resistance to earth of 7.8 ohms (assuming 10,000 ohm-cmsoil). Nine 3 m (10 ft.)ground rods installed 3 m (10 ft.)apart in the same 24.4 m (80 ft.)straight line

will achieve a resistance to earth of 5.7 ohms.

N

apart in an)ground rods installed 6.1 m (20 ft.)apart from one another). For example, five 3 m (10 ft.

will achieve a lower resistance to earth than fewer rods installed further apart (such as twice the length

In a given area, more ground rods installed closer together (such as one length apart from one another):OTE

4.4.1.2 GROUNDRODS

Typical ground rods are shown in Figure 4-7. Requirements for ground rods are listed below. See IEEE

STD 142-1991, section 4.3.1 and UL 467-2004 for additional information.

FIGURE 4-7 TYPICALGROUNDRODS

4.4.1.2.1 GROUNDRODSPECIFICATIONS

Ground rods shallbe UL listed (or equivalent).

Ground rods shallbe constructed of copper-clad steel, solid copper, hot-dipped galvanized steel, or

stainless steel (ANSI-J-STD-607-A-2002, section C.4.3, and ANSI T1.334-2002, section 5.3.2).

See NFPA 70-2005, Article 250.52 and UL 467-2004, section 9.2.1 for additional information.

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NOTE: Stainless steel ground rods shallbe formed of an austenitic stainless steel of the 18 percent chromium, 8percent nickel type (UL 467-2004, section 9.2.6).

Ground rods shallhave a minimum length of 2.4 m (8 ft.) (ANSI T1.313-2003, section 10.3.1,

ANSI T1.334-2002, section 5.3.2, NFPA 70-2005, Article 250.52, and UL 467-2004). For areas

highly prone to lightning, and/or military installations, longer rods, such as 3 m (10 ft.), should be

considered for the minimum length (MIL-HDBK-419A and MIL-STD-188-124B).

Ground rods shall have a minimum diameter of 15.9 mm (0.625 in.) (ANSI T1.313-2003, section

10.3.1 and ANSI T1.334-2002, section 5.3.2), unless otherwise allowed by the UL listing of the

ground rod (UL 467). See NFPA 70-2005, Article 250.52 for additional information.

Ground rods shall be free of paint or other nonconductive coatings (NFPA 70-2005, Article 250.53

and NFPA 780-2004, section 4.13.2).

4.4.1.2.2 GROUNDRODINSTALLATION

Where practical, ground rods shallbe buried below permanent moisture level (MIL-HDBK-419A

and NFPA 70-2005, Article 250.53).

Where practical, ground rods shallpenetrate below the frost line (MIL-HDBK-419A).

Ground rods longer than the minimum required 2.4 m (8 ft.) may be required to maintain contact

with permanently moist, unfrozen soil (MIL-HDBK-419A).

When part of a ground ring system, the upper end of the ground rods shall be buried to the depth of

the ground ring, typically 762 mm (30 in.) minimum below finished grade. The upper end of the

ground rods should be buried to the same depth as the ground ring to allow for easy bonding to the

ground ring. (See External Building and Tower Ground Ring on page 4-22.)

When not part of a ground ring system, such as in a Type A site, the entire length of the rod shallbe

in contact with soil (NFPA 70-2005, Article 250.53). It is recommended to install the ground rods

so the upper end of the rod is buried to a minimum depth of 610 mm (24 in.) below the surface of

the earth (NFPA 780-2004, section 4.13.2.3). See Figure 4-8for typical single ground rod

installations.

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REQUIREMENTS

GROUNDING CONDUCTORGROUNDING CONDUCTOR

TOP OF ROD JUSTBELOW SOIL

SURFACE

610 mm(2 ft)

3 m(10 ft.)

.4m

(8 ft.)

2.4 m(8 ft.)

RECOMMENDED DEPTH MINIMUM DEPTH

FIGURE 4-8 TYPICALSINGLEGROUNDRODINSTALLATION

Ground rods shall notbe installed closer than 1.8 m (6 ft.) from other ground rods and grounding

electrodes (NFPA 70-2005, Article 250.56). See Figure 4-31 on page 4-49for an example.

Unless otherwise stated in this chapter, ground rods shall not be installed closer to one another than

the sum of their respective lengths, when possible. This is especially important for the ground rodsassociated with tower ground rings. See Grounding (Earthing) Electrode Resistance

Characteristics and Sphere of Influence on page 4-9

See External Building and Tower Ground Ring on page 4-22for ground rod installation

requirements on ground rings.

The method of bonding grounding conductors to ground rods shallbe compatible with the types of

metals being bonded (See Dissimilar Metals and Corrosion Control on page 4-34).

Ground rods that cannot be driven straight down, due to contact with rock formations, may be

driven at an oblique angle of not greater than 45 degrees from the vertical, or may be buried

horizontally and perpendicular to the building, in a trench at least 762 mm (30 in.) deep, as shown

in Figure 4-9(NFPA 70-2005, Article 250.53).

IMPORTANT:The top of a ground rod shall notbe cut off if contact with rocks prevents driving of the

rod. Alternate driving techniques, as describe above, shallbe used in these cases.

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BURIED GROUNDING ELECTRODECONDUCTORS

2.4 m(8 ft)

45

maximum

76 cm

(2.5 ft)

FIGURE 4-9 ANGLEDGROUNDRODINSTALLATION

WARNING

When operating any kind of power tool, always wear appropriate safety glasses and

other protective gear to prevent injury.

Hammer drills or electric jackhammers may be used to drive in the ground rods. Do not deform the

head of the ground rod. See IEEE STD 142-1991, section 4.3.2. for additional information.

If rock formations prevent ground rods from being driven to the specified depth, an alternate

method of achieving an acceptable grounding electrode system shallbe engineered and

implemented. See Special Grounding (Earthing) Situations on page 4-88for additional

information.

When the grounding electrode system design requires deeper ground rods (in order to lower the

grounding electrode system resistance, penetrate down to permanent moisture level, or to penetrate

below the frost line) two or more ground rods may be joined together by use of a coupling

(threaded, compression sleeve, or exothermic weld). Threaded rods or compression sleeves shallbe

UL listed. (IEEE STD 142-1991, section 4.3.1). See Figure 4-10for an example of splicingground rods together.

FIGURE 4-10 SPLICINGTWOGROUNDRODS

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REQUIREMENTS

4.4.1.2.3 EFFECT OFGROUNDRODSIZE ONRESISTANCE TOEARTH

Increasing the diameter of a ground rod does not significantly reduce its resistance to earth. Doubling

the diameter of a rod reduces its resistance to earth by approximately 10%. See Figure 4-11.

0

5

10

15

20

25

30

35

40

45

50

A

pproximateResistancetoEarthinOhms

0.50 0.75 1.00 1.25 1.50 1.75

Ground Rod Diameter (Inches)

3 m (10 ft) Ground Rod in 10,000 ohm-cm soil

FIGURE 4-11 RESISTANCE TOEARTHDUE TOGROUNDRODDIAMETER

As the length of a ground rod is increased, its resistance to earth is substantially reduced. In general,

doubling the length of a ground rod reduces the resistance to earth by 40%. See Figure 4-12.

140

120

ApproximateResistancetoEarthinOhms

100

80

60

40

20

0

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Ground Rod Length (Feet)

15.9 mm (0.625 in.) Diameter Ground Rod in 10,000 ohm-cm soil

FIGURE 4-12 RESISTANCE TOEARTHDUE TOGROUNDRODLENGTH

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4.4.1.2.4 EFFECT OFPARALLELGROUNDRODS

Figure 4-13below shows the effects of adding additional ground rods (15.9 mm (0.625 in.) diameter by

3 m (10 ft.)long) together in parallel. As seen in the figure, the addition of one ground rod to the first

ground rod (for a total of two rods) significantly reduces the resistance to earth of the ground rod

system. Each subsequent ground rod added in parallel has less of an effect on the resistance to earth of

the ground rod system.


35

30

25

20

15

10

5

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Number of 3 m (10 ft) Ground Rods

Installed 6.1 m (20 ft) apart in 10,000 ohm-cm soil

FIGURE 4-13 RESISTANCE TOEARTHDUE TOPARALLELGROUNDRODS

4.4.1.3 ELECTROLYTICGROUNDRODS

At sites where, due to poor soil conductivity (high resistivity) and/or limited space, an acceptable

grounding (earthing) electrode system resistance cannot be achieved using standard ground rods,

commercially available electrolytic ground rods should be considered. See MIL-HDBK-419A Volume

I, section 2.9.5, and UL 467-2004, section 9.2.7 for additional information. Electrolytic ground rods

(Figure 4-14) are available in straight or L-shaped versions and in various lengths from 3 m (10 ft.) to

6.1 m (20 ft.), or longer as a special order. Electrolytic ground rods are generally constructed of 54 mm

(2.125 in.) diameter hollow copper pipe. This copper pipe is filled with a mixture of non-hazardous

natural earth salts. Holes at various locations on the pipe allow moisture to be hygroscopically extracted

from the air into the salt within the pipe, therefore forming conductive electrolytes. These electrolytes

then leach out of the pipe into the soil, improving soil conductivity. Electrolytic ground rods are insertedinto a pre-drilled hole, or in the case of L-shaped rods, placed into a trench at least 762 mm (30 in.)

deep, and encased in a grounding electrode encasement material. See Grounding (Earthing) Electrode

Encasement Materials on page 4-27.

Electrolytic ground rods should be considered for use in grounding electrode systems covered by

concrete or pavement, such as parking lots. By allowing moisture to enter, the design of the electrolytic

ground rod improves the resistance of the grounding electrode system.

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REQUIREMENTS

NOTE: Unless prohibited by local environmental authorities, condensation from the site's HVAC system maybe routed to the ground rod area to keep the soil moist, improving conductivity.

Electrolytic ground rods may provide significant improvement over standard ground rods of the same

length and may last several years longer than standard ground rods. The resistance to earth of

electrolytic ground rods is generally more stable in environments with variations in temperature and

moisture.

Requirements for the use of electrolytic ground rods are listed below:

Electrolytic rods shallbe UL listed (or equivalent).

Electrolytic rods shallbe installed per the manufacturers' recommendation.

Electrolytic rods shall be free of paint or other nonconductive coatings (NFPA 70-2005, Article

250.53 and NFPA 780-2004, section 4.13.2.2).

Electrolytes within the rod shallbe environmentally safe and approved by the environmental

authority having jurisdiction.

L-shaped electrolytic rods shallbe installed perpendicular to the building or shelter.

L-shaped electrolytic rods (horizontal portion) shallbe installed at least 762 mm (30 in.) below theearth's surface.

Grounding electrode encasement materials (also known as backfill) shallbe environmentally safe

and approved by the environmental authority having jurisdiction. See Grounding (Earthing)

Electrode Encasement Materials on page 4-27.

Electrolytic rods should be maintenance free.

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REQUIREMENTS

4.4.1.4 GROUNDPLATEELECTRODES

Ground plates (Figure 4-15) may be used in special cases, or if specifically engineered into the design

of the grounding (earthing) electrode system. Requirements for the use of ground plate electrodes are

listed below:

Ground plates should only be used if soil conditions prohibit the use of standard ground rods, or if

specifically engineered into the grounding electrode system.

Ground plates should be UL listed (or equivalent).

Ground plates shallbe constructed of copper or copper-clad steel.

Ground plates shallexpose not less than 0.37 m2(2 ft.2) of surface to exterior soil (MIL-HDBK

419A, section 2.5.5; NFPA 70-2005, Article 250.52; and NFPA 780-2004, section 4.13.6.1).

Ground plates shallhave a minimum thickness of 1.5 mm (0.06 in.) (MIL-HDBK-419A, section

2.5.5 and NFPA 70-2005, Article 250.52).

Ground plates shallbe free of paint or other nonconductive coatings (NFPA 70-2005, Article

250.53 and NFPA 780-2004, section 4.13.2.2).

Ground plates shallbe buried not less than 762 mm (30 in.) below the surface of the earth (NFPA

70-2005, Article 250.53). If soil conditions do not allow the ground plate to be buried at this depth,

see Shallow Topsoil Environments on page 4-97for additional information.

Where practical, a ground plate shallbe embedded below permanent moisture level

(BS 7430:1998, clause 10 and NFPA 70-2005, Article 250.53).

Ground plates should be installed vertically to allow for minimum excavation and better contact

with the soil when backfilling (BS 7430:1998, clause 10 and IEEE STD 142-1991, section 4.2.4).

See Figure 4-16.

STRAIGHT EDGESSERRATED EDGES PROVIDE MORE

EDGE SURFACE

FIGURE 4-15 TYPICALGROUNDPLATES

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GROUND PLATE

1.5 mm0.06 in.

#2 AWG BARECOPPER WIRE T

R ND RINMINIMUM)

BACKFILL MA ERIAL152 mm MINIMUM 6 IN.

ON ALL SIDES

EXOTHERMIC

CONNECTION

T P ED E F PLATE

762 mm (30 IN.) BELOW GRADE

SITE SOIL

FIGURE 4-16 TYPICALGROUNDPLATEINSTALLATION

A grounding electrode encasement material should be used to backfill around the ground plate to

help insure effective contact with the earth (BS 7430:1998, clause 10). See Grounding (Earthing)

Electrode Encasement Materials on page 4-27.

4.4.1.5 CONCRETE-ENCASEDELECTRODES

Though concrete-encased electrodes (also known as Ufer electrodes, named after Herbert G. Ufer, or

foundation earth electrodes) are not required by this standard, they should be used in new construction

as a method of supplementing the grounding (earthing) electrode system (IEC 61024-1-2, section 3.3.5).

Concrete-encased electrodes (Figure 4-17) enhance the effectiveness of the grounding electrode system

in two ways: the concrete absorbs and retains moisture from the surrounding soil, and the concrete

provides a much larger surface area in direct contact with the surrounding soil. This is especially helpful

at sites with high soil resistivity and/or limited area for installing a grounding electrode system. See

IEEE STD 142-1991 section 4.2.3, and the International Association of Electrical Inspectors

publication, Soares Book on Grounding and Bonding, 9th Edition, Appendix A for additional

information. Requirements for a concrete-encased electrode, if used, are listed below (IEC 61024-1-2;

NFPA 70-2005, Article 250.52; and NFPA 780-2004, section 4.13.3).

Concrete-encased electrodes shall be encased by at least 51 mm (2 in.) of concrete, located within

and near the bottom of a concrete foundation or footing that is in direct contact with the earth.

Concrete-encased electrodes shall be at least 6.1 m (20 ft.) of bare copper conductor not smaller

than 25 mm2csa (#4 AWG) or at least 6.1 m (20 ft.) of one or more bare or zinc galvanized or other

conductive coated steel reinforcing bars or rods at least 12.7 mm (0.5 in.) in diameter.

Concrete-encased electrodes shallbe bonded to any other grounding electrode system at the site.

See Common Grounding (Earthing) on page 4-5.

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REQUIREMENTS

25 mm2CSA (#4 AWG) OR

COARSER BARE COPPER GROUNDING ELECTRODECONDUCTOR OR STEEL CONDUCTORREINFORCING BAR OR ROD, NOTLESS THAN 12.7 mm (0.5 in.) NONMETALLIC

DIAMETER AND AT LEAST 6.1 m PROTECTIVE SLEEVE(20 ft) LONG

CONNECTION LISTEDFOR THE PURPOSE

51 mm(2 in.)

MINIMUM

FOUNDATION IN DIRECT CONTACT WITH EARTH

CLAMP SUITABLE FOR ENCASEMENT

OR EXOTHERMIC WELD

MINIMUM 6.1 m (20 ft)

END VIEWSIDE VIEW

225 mm csa (#4 AWG)

COPPER CONDUCTOR

MINIMUM 6.1 m (20 ft)

SIDE VIEW END VIEW

12.7 mm (0.5 in.) REBAR

(TYPICAL)

FIGURE 4-17 TYPICALCONCRETE-ENCASEDELECTRODES

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4.4.1.6 EXTERNALBUILDING ANDTOWERGROUNDRING

The buried external ground rings (building and tower) provide a means of bonding ground rods together

and bonding other grounding (earthing) electrode system components together, improving the overall

grounding electrode system. The ground rings also help to equalize potential in the earth surrounding

the tower and building structures, regardless of earth resistivity, by insuring that a low impedance

current path exists throughout the area (ANSI T1.334-2002, section 5.3).

Requirement for external ground rings are listed below (see Figure 4-18):

Unless otherwise stated, ground ring conductors shallbe 35 mm2csa (#2 AWG) or coarser, bare,

solid, tinned or un-tinned, copper (ANSI T1.313-2003 and ANSI T1.334-2002, section 5.3.1). See

Grounding (Earthing) Conductors on page 4-28for grounding conductor specifications.

Solid, bare, tinned, copper conductor should be used to minimize galvanic corrosion between tower

legs and other parts of the grounding electrode system (ANSI T1.313-2003, section 10.7).

For areas highly prone to lightning, and/or military installations, larger conductors, such as

50 mm2 csa (#1/0 AWG) or coarser, should be considered (MIL-HDBK-419A and MIL-STD-188

124B); stranded conductors may be used in this application.

Building ground rings shallencircle the building or shelter whenever possible (ANSI T1.313-2003,ANSI T1.334-2002, MIL-HDBK-419A, and MIL-STD-188-124B).

Tower ground rings shallencircle the tower structure whenever possible (ANSI T1.334-2002,

section 5.3 and MIL-HDBK-419A).

The ends of the conductor shallbe joined together to form a ring using an exothermic weld or listed

irreversible high-compression connector (ANSI T1.334-2002, section 5.3.1 and MIL-STD-188

124B). This may be easily completed at a ground rod.

Building ground rings and tower ground rings shall be bonded together in at least two points using

a 35 mm2 csa (#2 AWG) or coarser, bare, solid, tinned or un-tinned, copper conductor (ANSI-J

STD-607-A-2002, section C.4.7, ANSI T1.334-2002, figure 1, and MIL-STD-188-124B). The

conductors should be physically separated as much as practical. See Common Grounding

(Earthing) on page 4-5.

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REQUIREMENTS

FIGURE 4-18 BONDINGBUILDING ANDTOWERGROUNDRINGSYSTEMS

Ground rings shallbe installed in direct contact with the earth at a depth of 762 mm (30 in.) below

the earth's surface whenever possible, or below the frost line, whichever is deeper (ANSI T1.334

2002, section 5.3.1 and NFPA 70-2005, Article 250.53).

Building ground rings shallbe installed at least 914 mm (3 ft.) from the building foundation and

should be installed beyond the drip line of the roof. It is recommended that the building ground ring

and ground rods be positioned 610 mm to 1.8 m (2 ft. to 6 ft.) outside the drip line of the building or

structure to ensure that precipitation wets the earth around the ground ring and rods (MIL-HDBK

419A and MIL-STD-188-124B).

Tower ground rings shallbe installed at least 610 mm (2 ft.) from the tower foundation (ANSI

T1.334-2002, section 5.3.1).

If 2.4 m (8 ft.)ground rods are installed along the ground rings, they shallbe connected to the

ground ring conductor at 3 m to 4.6 m (10 ft. to 15 ft.)intervals (ANSI T1.334-2002), unless

otherwise specified.

If longer ground rods are used, a larger separation proportional to the increase in rod length

may be used.

Ground rods shallbe placed a minimum of one rod length apart from one another along the

ground rings (ANSI T1.313-2003, figure 3(a)).

Ground rods shall notbe separated from an adjacent ground rod along the ground ring by more

than the sum of their respective lengths. (MIL-HDBK-419A).

TOWER RING BONDED TO

BUILDING RING IN AT

LEAST 2 PLACES

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4.4.1.7 RADIAL(COUNTERPOISE) GROUNDINGCONDUCTORS

For high lightning prone geographical areas, sites normally occupied (such as 911 dispatch centers),

sites with high soil resistivity, or when bedrock prohibits the driving of ground rods, radial

(counterpoise) grounding (earthing) conductors should be employed to improve equalization of the

grounding electrode system (ANSI T1.334-2002, section 5.4), and to help meet the site's grounding

electrode system resistance requirements (see Grounding (Earthing) Electrode System Resistance

Requirements on page 4-46). Radial grounding conductors are conductors installed horizontally in the

ground and radiating away from the tower and building.

In typical soil resistivity conditions of 10, 000 ohm-cm, the addition of five radial conductors 7.6 m

(25 ft)in length may reduce the tower grounding electrode system resistance by a factor of two or three.

More importantly, adding radial conductors divides lightning strike current into segments that allow for

more effective dissipation of energy into the earth, and away from the equipment building.

When used, radial conductors shall meet the following specifications:

The conductors shallradiate away from the building and tower (ANSI T1.334-2002, section 5.4).

The conductors shallbe installed at the tower or tower ground ring whenever possible. If the

conductors cannot be installed at the tower, installation at the building is acceptable, but should be

installed near the RF transmission line entry point.

FIGURE 4-19 INSTALLATION OFRADIALCONDUCTORS

When radial conductors are used, a minimum of three to five conductors should be used.

The conductors shall be installed equally spaced from one another, as much as practical.

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----------------------


REQUIREMENTS

The conductors shallbe bonded directly to the tower and tower ground ring (ANSI T1.334-2002,

section 5.4). If it is not practical to bond all conductors to the tower, the tower shallhave additional

grounding conductors bonding it to the tower ground ring; 70 mm2(#2/0 AWG) or coarser

conductor is recommended in this case.

Conductor bonding shallcomply with Bonding to the External Grounding (Earthing) Electrode

System on page 4-40

The conductors shallbe constructed of 35 mm2csa (#2 AWG) or coarser, bare, solid, tinned or un

tinned, copper. See Grounding (Earthing) Conductors on page 4-28for conductor specifications

and installation requirements. (ANSI T1.334-2002, section 5.4)

The conductors shallbe buried at least 457 mm (18 in.) below ground (ANSI T1.334-2002,

section 5.4). When topsoil conditions allow, it is recommended to bury the conductors to a depth of

at least 762 mm (30 in.) (ANSI-J-STD-607-A-2002, section C.9.2); this is especially important in

areas where the frost line may reach 457 mm (18 in.).

The minimum length of each conductor shallbe 7.6 m (25 ft.). If the desired resistance to earth is

not achieved at 7.6 m (25 ft.), the radial conductor may be extended to help obtain the desired

resistance (ANSI T1.334-2002, section 5.4). The maximum effective length for a single radial

conductor is generally considered to be approximately 24.4 m (80 ft.). Adding additional

conductors is generally more effective than extending the length of a single conductor.

NOTE: When multiple radial conductors are used, the conductors should be of different lengths to help preventresonant ringing of the tower from a lightning strike.

NOTE: Low resistance in radial (counterpoise) grounding configurations is desirable, but not critical. Lowresistance in the dissipating path of strike currents into the earth is of secondary importance when

compared to the primary objective of controlling voltage gradients and voltage differences between

structures and equipment close to the tower (ANSI T1.334-2002, section 5.4).

When soil conditions allow, the effectiveness of the radial grounding conductor may be increasedby including a ground rod every 4.9 m (16 ft.) (or twice the length of the ground rods) installed as

described in Ground Rods on page 4-11. See Figure 4-19 on page 4-24for an example of ground

radials.

Figure 4-20 on page 4-26shows the resistance characteristics of a radial grounding conductor. The

resistance to earth of a straight horizontal electrode (radial grounding conductor) may be calculated as

follows:

1 2LR = ------ ln

)1 2L (2aD /

Where:

D


26/100



Ap

proximateResistancetoEarthinOhms

40

35

30

25

20

15

10

5

0

10 20

40

35

30

25

20

15

10

5

0

Radial Grounding Conductor Resistance

AWG # 2

Buried 457 mm (18 in.) in 10,000 ohm-cm soil


30 40 50 60 70 80

Radial Grounding Conductor Length (feet)

Radial Grounding Conductor Resistance

AWG # 2/0



10 20 30 40 50 60 70 80

Radial Grounding Conductor Length (feet)

FIGURE 4-20 RESISTANCECHARACTERISTICS OF ARADIALGROUNDINGCONDUCTOR

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Grounding electrode encasement material shallbe packaged for the purpose of grounding electrode

encasement.

Grounding electrode encasement material shallbe environmentally safe and approved by the

environmental authority having jurisdiction.

Grounding electrode encasement material shallbe used in accordance with the manufacturers'

instructions.

Grounding electrode encasement material shall nothave a corrosive effect on the grounding

electrode system components.

The use of charcoal or petroleum based coke breeze is not recommended as it may result in rapid

corrosion of copper electrodes and copper conductors (BS 7430:1998, clause 8.5; BS 6651:1999,

clause 18.4.2; and FAA STD 019d-2002, section 3.8.3.5). Charcoal and petroleum based coke

typically contains high levels of sulfur, which in the presence of moisture will accelerate corrosion.

Coke breeze derived from coal in coke ovens is generally considered acceptable; all the corrosives

and volatiles have been cooked off at extremely high temperatures (FAA STD 019d-2002, section

3.8.3.5).

Per MIL-HDBK-419A, the suggested grounding electrode encasement (backfill) material is a mixture

of 75 percent gypsum, 20 percent bentonite clay, and 5 percent sodium sulfate. The gypsum, which is

calcium sulfate, absorbs and retains moisture and adds reactivity and conductivity to the mixture. Since

it contracts very little when moisture is lost, it will not pull away from the ground rod or surrounding

earth. The bentonite ensures good contact between the ground rod and earth by its expansion, while the

sodium sulfate prevents polarization of the ground rod by removing the gases formed by current

entering the earth through the ground rod. This mixture is readily available from cathodic protection

distributors as standard galvanic anode backfill. The backfill mixture should be covered with 305 mm

(12 in.) of excavated soil. See MIL-HDBK-419A Volume I, section 2.9 for additional information.

4.4.2 GROUNDING(EARTHING) CONDUCTORS

Grounding (earthing) conductors are the conductors used to connect equipment or the grounded circuitof a wiring system to a grounding electrode or grounding electrode system. These conductors may

connect grounding electrodes together, form buried ground rings, and connect objects to the grounding

electrode system. See BS 7430:1998, clause 3.17 and NFPA 70-2005, Article 100 for additional

information.

4.4.2.1 GENERALSPECIFICATIONS

General specifications for grounding (earthing) conductors are listed below.

Unless otherwise stated, all below-ground, or partially below-ground, external grounding electrode

system conductors shallbe 35 mm2 csa (#2 AWG) or coarser, bare, solid, tinned or un-tinned,

copper conductors (ANSI T1.313-2003 and ANSI T1.334-2002, section 5.3). For areas highlyprone to lightning, and/or military installations, larger conductors, such as 50 mm2csa (#1/0 AWG)

or coarser, should be considered (MIL-HDBK-419A); stranded conductors may be used in this

application. Tinned conductors are recommended for stranded conductors.

Solid, bare, tinned, copper conductors should be used to help minimize galvanic corrosion between

tower legs and other parts of the grounding electrode system (ANSI T1.313-2003, section 10.7).

See Dissimilar Metals and Corrosion Control on page 4-34.

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REQUIREMENTS

Grounding electrode conductors shall be installed in one continuous length without a splice or

joint, unless spliced using irreversible compression-type connectors listed for the purpose or by

exothermic welding (NFPA 70-2005, Article 250.64). See Bonding Methods on page 4-41for

additional information.

Above-ground conductors used for bonding individual metallic objects shallbe 16 mm2csa (#6

AWG) or coarser, tinned or un-tinned, copper conductors (ANSI T1.334, section 5.3.3). See

Metallic Objects Requiring Bonding on page 4-67for additional information.

Above-ground conductors used for bonding multiple metallic objects (used as a ground bus

conductor) shallbe 35 mm2csa (#2 AWG) or coarser, tinned or un-tinned, copper conductors. See

Metallic Objects Requiring Bonding on page 4-67.

Above-ground bonding conductors should be jacketed, whenever practical (ANSI T1.334-2002,

section 5.1).

Solid straps or bars may be used as long as the cross-sectional area equals or exceeds that of the

specified grounding conductor.

WARNING

The AC power system grounding conductors shall be sized appropriately for the

electrical service and shall be approved by the authority having jurisdiction.

4.4.2.2 BENDINGANDROUTINGGROUNDING(EARTHING) CONDUCTORS

Grounding (earthing) conductors shall be run in a direct manner with no sharp bends or narrow loops

(ANSI T1.313-2003, section 11.3, and ANSI T1.334-2002, section 13.4). Sharp bends and/or narrow

loops increase the impedance and may produce flash points (also see NFPA 780-2004, section 4.9.5).

The following requirements apply when installing grounding system conductors:

Sharp bends shallbe avoided (ANSI T1.334-2002, section 13.4).

Grounding conductors shallbe run as short, straight, and smoothly as possible, with the fewest

possible number of bends and curves (ANSI T1.313-2003, section 11.3; ANSI T1.334-2002,section 13.4; and NFPA 70-2005, Articles 800.100, 810.21, and 820.100).

A minimum bending radius of 203 mm (8 in.) shallbe maintained, applicable to grounding

conductors of all sizes (ANSI T1.313-2003, section 11.3; MIL-STD-188-124B; and NFPA 780

2004, section 4.9.5). A diagonal run is preferable to a bend even though it does not follow the

contour or run parallel to the supporting structure. See Figure 4-22.

All bends and curves shall be made toward the ground location (grounding electrode system or

ground bar).

The radius of any bend shall notbe less than 203 mm (8 in.)

Radius

90 degrees

minimum

The angle of any bend shall notbe less than 90 degrees.

FIGURE 4-22 MINIMUMBENDINGRADIUSFORGROUNDINGCONDUCTORS

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4.4.2.3 PROTECTING ANDSECURINGGROUNDING(EARTHING) CONDUCTORS

Above ground external grounding (earthing) conductors, including straps, are exposed to movement by

wind and other physical forces that can lead to damage or breakage over time. The following

requirements shallapply when installing grounding conductors:

Grounding conductors shallbe protected where exposed to physical damage (NFPA 70-2005,

Articles 250.64, 800.100, 810.20, 820.100; and NFPA 780-2004, section 4.9.11).

Grounding conductors exposed to physical damage shall be protected for a minimum distance of

1.8 m (6 ft.) above grade level (NFPA 780-2004, section 4.9.11.2). Such areas may include, but are

not limited to, runways, driveways, school playgrounds, cattle yards, public walks (NFPA 780

2004, section 4.9.11).

Metallic guards and/or conduits used to protect grounding conductors shallbe bonded to the

grounding conductor at both ends (NFPA 70-2005, Article 250.64 and NFPA 780-2004, section

4.9.11.1).

The grounding conductor or its enclosure shall be securely fastened to the surface on which it is

carried (NFPA 70-2005, Articles 250.64 and 810.21; and NFPA 780-2004, section 4.10).

Grounding conductors shallbe secured using appropriate hardware intended for the purpose.

When metallic fasteners are used on bare grounding conductors, fasteners of the same material

shallbe used, or approved bonding techniques shallbe observed for the connection of dissimilar

metals. See Dissimilar Metals and Corrosion Control on page 4-34. See NFPA 780-2004, section

4.10.2 for additional information.

Above ground grounding conductors shall be securely fastened at intervals not exceeding 91 cm

(3 ft.) where practical. (ANSI T1.334-2002, section 8.3 and NFPA 780-2004, Section 4.10)

4.4.3 EXTERNALGROUNDBUSBAR

The purpose of the external ground bus bar (EGB) is to provide a convenient grounding (earthing)

termination point for antenna transmission lines (coaxial cables) and other cables prior to their entry

into a building or shelter (ANSI T1.313-2003). Antenna transmission lines and other communications

cables with metallic sheaths shallbe grounded as close as practical to their point of entry into the

building or shelter (NFPA 70-2005, articles 770.93, 800.100, 810.20, 820.93, and 820.100).

Requirements for external ground bus bars, when used, are listed below:

The EGB shall be constructed and minimally sized in accordance with Table 4-3 on page 4-32,

ensuring the ground bus bar is large enough to accommodate all transmission lines and other

grounding connections.

The EGB shall be designed for the purpose of grounding and should be UL listed,

The EGB shallbe installed at the point where the antenna transmission lines and other

communications cables enter the building or shelter.

The EGB shallbe connected directly to the grounding electrode system using a downward run of

35 mm2csa (#2 AWG) or coarser, bare, solid or stranded, tinned or un-tinned, copper conductor; it

is recommended to use a larger conductor, such as 120 mm2(#4/0 AWG) (United States National

Weather Service Manual 30-4106-2004, Lightning Protection, Grounding, Bonding, Shielding,

and Surge Protection Requirements). See Figure 4-23. The grounding conductor shallbe installed

in a direct manner with no sharp bends or narrow loops. (See Bending And Routing Grounding

(Earthing) Conductors on page 4-29.)

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TABLE 4-3 EXTERNALGROUNDBUSBARSPECIFICATIONS(WHENREQUIRED)

Item Specification

Material Bare, solid Alloy 110 (99.9%) copper bus bar or plate of one piece

construction. May be tin-plated.

Minimum Dimensions

(ANSI-J-STD-607-A-2002 and NFPA 70-2005, Article 250.64)

Height: 51 mm (2 in.)

Thickness: 6.35 mm(0.25 in.)

Length: Variable to meet the application requirements and allow

for future growth. 305 mm (12 in.) is recommended as the

minimum length.

Mounting brackets Stainless steel

Insulators Polyester fiberglass 15 kV minimum dielectric strength flame

resistant per UL 94 VO classification

Conductor mounting holes Number dependent on number of conductors to be attached

Holes to be 11 mm (0.4375 in.) minimum on 19 mm (0.75 in.)

centers to permit the convenient use of two-hole lugs.

Method of attachment of grounding electrode conductor Exothermic welding

Irreversible crimp connection

IMPORTANT: For improved lightning protection at the site, the RF transmission line entry point andEGB should be installed as low to the ground as practical; 610 mm (2 ft.) is therecommended maximum height for the RF transmission line entry point. (United States

National Weather Service Manual 30-4106-2004, Lighting Protection, Grounding,Bonding, Shielding, and Surge Protection Requirements.) See Design Considerations

to Help Reduce Effects of Lightning on page 2-19.

4.4.3.1 TOWERGROUNDBUSBAR

The purpose of the tower ground bus bar (TGB) is to provide a convenient termination point on the

tower for multiple transmission line (coaxial) grounding (earthing) conductors. The tower ground bus

bar should be an integral part of the tower construction. If the tower ground bus bar is not part of the

tower construction, it shallbe constructed and minimally sized in accordance with

Table 4-4 on page 4-34, ensuring the ground bus bar is large enough to accommodate all coaxial cable

connections and connection to the grounding electrode system.

The requirements for installing tower ground bus bars are as follows:

Where a galvanized tower is not protected against precipitation run-off from copper and copperalloys, the tower ground bus bar (TGB) shallbe constructed of tinned copper. See Methods To

Help Reduce Corrosion on page 4-38.

The tower ground bus bar shallbe installed below the transmission line ground kits, near the area

of the tower at the point where the antenna transmission lines transition from the tower to the

shelter.

The tower ground bus bar shallbe connected to the tower grounding electrode system with a

35 mm2csa (#2 AWG) or coarser, bare, solid, tinned, copper conductor.

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REQUIREMENTS

For reduced impedance to earth, the tower ground bus bar may be directly bonded to the tower,

using hardware of materials suitable for preventing dissimilar metal reactions, if possible and

allowed by the tower manufacturer. This is in addition to the required grounding conductor as

described above.

The grounding conductors shallbe run as short, straight, and smoothly as possible. See Bending

And Routing Grounding (Earthing) Conductors on page 4-29.

The grounding conductor may be sleeved in PVC for protection if desired (ANSI T1.313-2003,

section 11.4). This may be required in order to keep the grounding conductor from making

incidental contact with the tower.

For reduced impedance to the grounding electrode system, the TGB can be connected to the external

grounding electrode system using solid copper strap. Relatively small copper strap has significantly less

inductance (impedance to lightning) than large wire conductors. For example, 38.1 mm (1.5 in.) copper

strap has less inductance than 70 mm2csa (#2/0 AWG) wire. To further reduce the inductance to

ground, several copper straps can be installed across the entire length of the tower ground bus bar and

routed down to the external grounding ring.

Additional ground bus bars may be installed at different heights along the vertical length of the tower

for bonding multiple transmission line ground kits to the tower, if not already included as part of the

tower structure. The additional ground bus bars shallbe bonded directly to the tower using tower

manufacturer approved methods. Bonding to the tower may include the following options:

Bolting a tin-plated bus bar directly to the tower structure using stainless steel hardware. In this

case, a grounding conductor is not required.

Securing a bus bar to the tower using appropriate mechanical hardware. Electrically bonding the

bus bar to the tower using a grounding conductor. The grounding conductor should bond to the

tower using appropriate hardware, such as stainless steel beam clamps, or stainless steel band/strap

type clamp. The grounding conductor shallbond to the bus bar using exothermic weld, irreversible

compression connectors, or listed compression two-hole lugs.

TOWER GROUND BUS BAR

TRANSMISSION LINESTO BUILDING

TOWER GROUND RING (UNDERGROUND)

FIGURE 4-25 TYPICALTOWERGROUNDBUSBARS

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DISSIMILARMETALS ANDCORROSIONCONTROL CHAPTER4: EXTERNALGROUNDING(EARTHING)

TABLE 4-4 TOWERGROUNDBUSBARSPECIFICATIONS

Item Specification

Material Bare, solid Alloy 110 (99.9%) copper bus bar or plate of one piece construction. Should be

tin-plated if installing on a galvanized tower. (See Dissimilar Metals and Corrosion

Control on page 4-34for information regarding tower corrosion related to copper bus bars.)

Minimum Dimensions

(ANSI-J-STD-607-A-2002 and NFPA

70-2005, Article 250.64)

Height: 51 mm (2 in.)

Thickness:6.35 mm(0.25 in.)

Length: Variable to meet the application requirements and allow for future growth.

305 mm (12 in.) is recommended as the minimum length.

Mounting brackets Stainless steel

Conductor mounting holes Number dependent on number of conductors to be attached

Holes to be 11 mm (0.4375 in.) minimum on 19 mm (0.75 in.) centers to permit the

convenient use of two-hole lugs.

Method of attachment of grounding

electrode conductor

Exothermic welding

Irreversible crimp connection

4.5 DISSIMILARMETALS ANDCORROSIONCONTROL

Although the type of metals used in a grounding (earthing) electrode system do not affect the resistance

to earth of the grounding electrode system, consideration should be given to select a metal that is

resistance to corrosion in the type of soil in which it will be installed. The two areas that should be

considered regarding the corrosion resistance of a metal are the compatibility with the soil itself and

possible galvanic corrosion effects when it is electrically connected to neighboring metals at the site.

(BS 7430:1998)

4.5.1 CORROSIONRELATED TOSOILTYPE

The compatibility of a metal with soil is determined by the chemical composition of the soil. The

chemical composition factors associated with the corrosion of metals in contact with the soil are as

follows: acidity or alkalinity (pH), salt content, soil porosity (aeration), and the presence of anaerobic

bacteria. (BS 7430:1998 and TIA/EIA-222-F-R2003)

The following list gives a general representation of the aggressiveness of soils, listed in order of

increasing aggressiveness (BS 7430:1998):

Gravelly soils (Least Aggressive)

Sandy soils

Silty soils (loam)

Clays

Peat and other organic soils

Made up soils containing cinders (Most Aggressive)

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STANDARDS ANDGUIDELINES FORCOMMUNICATIONSITES DISSIMILARMETALS ANDCORROSIONCONTRO

The least aggressive soils tend to be those having a high resistivity. The resistivity of soil can be

measured, which provides an indication of corrosiveness under aerated conditions (BS 7430:1998). See

Performing Soil Resistivity Test on page B-6 for measurement details. Soil with a resistivity below

2,000 ohm centimeters (cm) is generally considered to be highly corrosive (TIA/EIA-222-F-R2003)

More details about the aggressiveness of soils can be obtained by measuring the redox (from the words

reduction and oxidation) potential of the soil, which indicates the risk of corrosion due to the presence

of anaerobic bacteria (BS 7430:1998). Test equipment required to measure redox potential is

commercially available. The procedure required to test the redox potential can be found in ISO

11271:2002(E). A geotechnical firm may be required to measure the redox potential of the soil.

General guidance on the corrosiveness of some grounding electrode system metals in relation to soil

composition is given below in Table 4-5(BS 7430:1998). A geotechnical firm may be required to

determine all of the listed soil parameters.

TABLE 4-5 CORROSIONRESISTANCEPROPERTIES OFCOMMONGROUNDINGELECTRODESYSTEMMETALS ASRELATED

TOSOILCOMPOSITION

Soil Parameter

Grounding Electrode Metal

Copper Galvanized Steel

Austenitic Stainless

Steel*

Resistivity (cm)

< 700 Slightly Reduced Moderately Reduced Slightly Reduced

700 to 4000 Slightly Reduced Moderately Reduced Generally Unaffected

> 4000 Generally Unaffected Generally Unaffected Generally Unaffected

Redox Potential (mV)

< 200 Moderately Reduced Considerably Reduced Moderately Reduced

200 to 400 Slightly Reduced Slightly Reduced Generally Unaffected

> 400 Generally Unaffected Generally Unaffected Generally Unaffected

Moisture Content (%)

< 10 Generally Unaffected Generally Unaffected Generally Unaffected

10 to 80 Slightly Reduced Moderately Reduced Slightly Reduced

> 80 Slightly Reduced Slightly Reduced Slightly Reduced

Dissolved

Salts Moderately Reduced Moderately Reduced Slightly Reduced

Chlorides Moderately Reduced Moderately Reduced Moderately Reduced

pH

Acidic < 6 Moderately Reduced Considerably Reduced Slightly Reduced

Neutral 6 to 8 Generally Unaffected Generally Unaffected Generally Unaffected

Alkaline > 8 Slightly Reduced Moderately Reduced Generally Unaffected

Organic Acids > 8 Considerably Reduced Moderately Reduced Slightly Reduced

* Austenitic stainless steel shallbe formed from 18% chromium and 8% nickel (18/8 stainless steel), per UL 467-2004, section 9.2.6.Table based on information from BS 7430:1998.

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The following general observations can be made from Table 4-5:

Copper-clad steel or solid copper ground rods are one of the better and commonly used materials

for grounding electrodes. However, the adverse effect of dissolved salts, organic acids and acid

soils generally should be noted (BS 7430:1998, clause 11).

Copper or copper-clad ground rods should not be used in soils where organic acids are present,

unless protective measures are taken, such as encasing the ground rods in a grounding electrodeencasement material. Organic acids are commonly found in poorly drained and poorly aerated soils.

See Grounding (Earthing) Electrode Encasement Materials on page 4-27.

Galvanized ground rods should not be used in soils with a redox potential below 200 mV, unless

protective measures are taken, such as encasing the ground rods in a grounding electrode

encasement material. See Grounding (Earthing) Electrode Encasement Materials on page 4-27.

Galvanized ground rods should not be used in acidic soils with a pH below 6.

4.5.2 GALVANICCORROSION

Galvanic corrosion (also called dissimilar metals corrosion) refers to corrosion damage induced when

two dissimilar metals are electrically connected and coupled through an electrolyte (such as soil). When

a metal is electrically connected to a dissimilar metal, a difference of potential exists between the two

metals. If the dissimilar metals are also in contact with a low resistivity soil, a complete circuit will

exist. Current will flow from one metal to the other due to the electrical connection and return path

through the soil. This naturally occurring phenomenon is why current is obtained from a battery when

its terminals are electrically connected to a load (TIA/EIA-222-F-R2003). See Figure 4-26for an

example of installations with and without galvanic corrosion.

COPPER NIZEDGROUND

ROD

GROUND ROD GROUND ROUND

ROD ROD

COPPER PER

FIGURE 4-26 INSTALLATIONSWITH ANDWITHOUTGALVANICCORROSION

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Metals may be listed in order of their respective potentials; such a list is called a galvanic series. A

galvanic series of commonly used grounding (earthing) electrode system metals and alloys is given in

Table 4-6(from TIA/EIA-222-F-R2003 and MIL-HDBK-419A). When a complete circuit exists,

corrosion occurs on the metal listed higher in the galvanic series. The metal listed higher in the galvanic

series (anode) is where current exits and travels through the soil toward the metal listed lower on the

galvanic series (cathode). The galvanic series of commonly used metals and alloys is as follows:

TABLE 4-6 GALVANICSERIES OFCOMMONMETALS

Anodic (Active) End

Magnesium

Zinc (material used to galvanize steel)

Aluminum

Steel, Iron

Lead, Tin

Brass, Copper, Bronze

Silver

Graphite

Cathodic (Most Noble) End

The rate of corrosion mainly depends on the conductivity of the soil and the relative position of the

metals in the galvanic series. The higher the soil conductivity (low resistivity), and the further apart themetals are in the galvanic series, the faster the rate of corrosion (TIA/EIA-222-F-R2003). To some

extent, the rate of corrosion also depends on the relative surface areas of the metals (BS 7430:1998 and

IEEE STD 142-1991). A small anode (such as a galvanized steel guy anchor point) and large cathode

(such as a copper grounding electrode system) should not be installed; in this case, the total current is

confined to a small space and the current density is large, therefore, corroding the galvanized steel

(IEEE STD 142-1991).

General guidance on the suitability of metals for bonding together with neighboring metals is given

below in Table 4-7(BS 7430:1998); both metals are assumed to be located in the earth. The bond

between the neighboring metals could be located above or below ground.

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TABLE 4-7 SUITABILITY OFMETALS FORBONDING

Metal assumed to have the Larger Surface Area

Electrode metal or item assumed to have the Smaller Surface

Area

Steel

Galvanized

Steel Copper

Tinned

Copper

Galvanized Steel Suitable Suitable Suitable Suitable

Steel in Concrete Not Suitable Not Suitable Suitable Suitable

Galvanized Steel in Concrete Suitable * Suitable Suitable

Lead Suitable * Suitable Suitable

Key:

Suitable = Materials suitable for bonding.

Not Suitable = Materials not suitable for bonding.

* = Materials suitable for bonding, but the galvanizing on the smaller surface may suffer.

This table is based on Table 8 of BS 7430:1998.

4.5.3 MISCELLANEOUSGENERALINFORMATION

Galvanized steel is strongly electronegative to copper and steel encased in concrete, therefore,

careful consideration must be give before galvanized ground rods are used at a site that contains a

concreted encased electrode. (See BS 7430:1998, clause 11.2 for additional information.) See

Concrete-Encased Electrodes on page 4-20.

Steel encased in concrete has a potential similar to that of copper; therefore, may be bonded tocopper or copper-clad ground rods. (See BS 7430:1998 and IEEE STD 142-1991 for additional

information.)

4.5.4 METHODSTOHELPREDUCECORROSION

Listed below are some general requirements and guidelines to help prevent corrosion of the grounding

(earthing) electrode system and other metallic items at the communications site:

The same metal shallbe used throughout the grounding electrode system whenever possible.

Aluminum or copper-clad aluminum grounding conductors shall notbe used (NFPA 70-2005,

Article 250.64).

Copper shall not come into incidental contact with galvanized steel.

Copper shall not come into incidental contact with aluminum.

Precipitation run-off from copper and copper alloys can attack galvanized parts (BS 6651:1999 and

IEC 61024-1-2, section 5.2); therefore, bare copper conductors or copper bus bars shall notbe

installed above galvanized steel, such as a tower, unless the steel is protected against the

precipitation run-off (IEC 61024-1-2, section 5.2).

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CAUTION

Extremely fine particles are shed by copper parts, which result in severe corrosivedamage to galvanized parts, even where the copper and galvanized parts are not in

direct contact (IEC 61024-1-2, section 5.2).

Where a galvanized tower is not protected against precipitation run-off from copper and copper

alloys, the tower ground bus bar (TGB) shallbe constructed of tinned copper or other suitable

material.

Where tinned conductors or galvanized ground rods are used, care shallbe exercised during

installation so that surfaces are not damaged. If surfaces of these ground elements are damaged, the

potential for deterioration from galvanic action increases (ANSI T1.313-2003, section 11.5 and

ANSI T1.334-2002, section 13.6).

Exothermically welded joints on galvanized material shall be coated with a zinc-enriched paint to

prevent corrosion.

Copper/aluminum joints shallbe avoided wherever possible. In cases where they cannot be

avoided, the connections shallbe exothermically welded or made using an AL/CU listed bimetallictransition connector (IEC 61024-1-2, section 5.2). Use a listed conductive anti-oxidant compound

on all mechanical connections (ANSI T1.334-2002, section 9).

Solid, bare, tinned copper conductor should be used to minimize galvanic corrosion between tower

legs and other parts of the grounding electrode system (ANSI T1.313-2003, section 10.7).

Grounding connections to galvanized towers shallbe exothermally welded whenever possible.

When exothermic welding is not possible, the tower grounding conductor shallbe constructed of

tinned-copper (IEC 61024-1-2, section 5.2).

Select appropriate grounding electrode system components using Table 4-5.

When soil conditions are not favorable, such as highly acidic or alkaline, grounding electrode

system component corrosion may be reduced by encasing the components in a grounding electrode

encasement material. See Grounding (Earthing) Electrode Encasement Materials on page 4-27,and BS 7430:1998, section 19.6.1, for additional information.

When soil conditions are not favorable, such as highly acidic or alkaline, grounding electrode

system component corrosion may be reduced by installing electrolytic ground rods encased in a

grounding electrode encasement material. See Grounding (Earthing) Electrode Encasement

Materials on page 4-27and Electrolytic Ground Rods on page 4-16.

When soil conditions are not favorable, such as highly acidic or alkaline, the useful life of a copper

ground rod can be extended by using solid copper ground rods instead of copper-clad rods, if soil

conditions allow driving of the solid copper rod.

When soil conditions are not favorable, such as highly acidic or alkaline, the useful life of buried

grounding conductors can be extended by using larger conductors, such as 70 mm2 csa (#2/0 AWG)

instead of 35 mm2(#2 AWG).

Use a listed conductive anti-oxidant compound on all mechanical connections (ANSI T1.334-2002,

section 9). The anti-oxidant compound shallbe liberally installed between the two metals (see

Figure 4-27 on page 4-40).

See Guy Anchor Points on page 4-54for information regarding proper grounding techniques to

help minimize galvanic corrosion of the guy anchor.

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BONDING TO THEEXTERNALGROUNDING(EARTHING) ELECTRODESYSTEM CHAPTER4: EXTERNALGROUNDING(EARTHING)

4.6 BONDING TO THEEXTERNALGROUNDING(EARTHING)ELECTRODESYSTEM

4.6.1 REQUIREMENTS

All below-grade grounding (earthing) electrode system connections shallbe joined using exothermic

welding or listed irreversible high-compression fittings compressed to a minimum of 13.3 tonnes

(12 tons) of pressure, or as otherwise required by the specific component manufacturer (ANSI T1.313

2003, figure 3(a)). Manufacturer requirements shall be followed for all connections. Connectors and

fitting used shallbe listed for the purpose, for the type of conductor, and for the size and number of

conductors used.

All above grade grounding electrode system connections (such as grounding electrode conductor

connection to ground bus bars and tower legs) shallbe joined using exothermic welding, or listed

irreversible high-compression fittings compressed to a minimum of 13.3 tonnes (12 tons) of pressure, or

as otherwise required by the specific component manufacturer (ANSI T1.313-2003, figure 3(a)).

All above grade bonding connections (such as bonding to ancillary equipment, or bonding coaxial

ground kits to bus bars) shallbe joined using exothermic welding, listed lugs, listed pressure

connectors, listed clamps, or other listed means required by the specific component manufacturer.

Connecting hardware shall be designed for the purpose, for the type of conductor, and for the size and

number of conductors used. All mechanical connections shall be coated with a listed conductive anti

oxidant compound. The anti-oxidant compound shall be liberally applied between the two metals (see

Figure 4-27)(NFPA 70-2005, Article 250.70).

METALLIC BONDINGAND APPLY ANTIOXIDANTCOMPOUND

REMOVE PAINT (IF PRESENT)

SURFACE

BOLT

NUT

GROUND LOCKWASHERLUG

GROUND BUS BAR

OR EQUIPMENT SURFACEGROUND LUG

LOCKWASHER

FIGURE 4-27 PROPERLOCATION OFWASHER WHENCONNECTINGGROUNDINGLUG

NOTE: In some instances, exothermic welding may not be possible or may be prohibited by the specificcomponent manufacturer (such as towers or fences); in these cases, other suitable means for bonding is

allowed.

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STANDARDS ANDGUIDELINES FORCOMMUNICATIONSITES BONDING TO THEEXTERNALGROUNDING(EARTHING) ELECTRODESYSTEM

Connecting hardware shallbe listed for the purpose, for the type of conductor, and for the size and

number of conductors used. All mechanical connections shall be coated with a listed conductive anti

oxidant compound (NFPA 70-2005, Article 250.70).

All exothermic and irreversible compression connections for use on external grounding applications

shallbe UL 467 listed. Copper connectors shallmaintain minimum 88% conductivity rating.

Compression systems shallinclude crimped die index and company logo for purposes of inspection.

Aluminum shall notbe used for connection purposes.

Bonding shallbe performed so that a suitable and reliable connection exists. The following

requirements shall be observed when bonding grounding connections:

Paint, enamel, lacquer and other nonconductive coatings shallbe removed from threads and

surface areas where connections are made to ensure good electrical continuity (NFPA 70-2005,

Article 250.12). Use of a star washer does not alleviate the requirement to remove nonconductive

coatings from attachment surfaces. See Figure 4-27for proper star/lock washer location. Star

washers should only be used as a lock washer.

After bonding to a painted or galvanized structure, the area shallbe painted with a zinc-enriched

paint.

Exothermic welding is the preferred method for bonding connections to the external groundingelectrode system.

Two-hole lugs secured with fasteners in both holes are preferred over single-hole lugs. Two-hole

lugs prevent movement of the lug.

When connecting ground lugs or compression terminals to ancillary equipment, such as air

conditioners and vent hoods, a lock washer shallbe placed on the nut side. See Figure 4-27. Sheet

metal screws and/or self-tapping screws shall notbe used.

All mechanical connections shallbe coated with a listed conductive anti-oxidant compound (NFPA

70-2005, Article 250.70, ANSI T1.334-2002, section 9). The anti-oxidant compound shallbe

liberally installed between the two metals (see Figure 4-27 on page 4-40).

4.6.2 BONDINGMETHODS

The following paragraphs describe acceptable methods for bonding to the external grounding (earthing)

electrode system. Exothermic welding and the use of listed irreversible high-compression fittings are

the only acceptable methods for below-grade bonding. Other mechanical connection methods shall not

be used below-grade.

4.6.2.1 EXOTHERMICWELDING

Exothermic welding is a method of welding electrical connections without an external heat source, such

as electricity or gas. The process is based on the reaction of granular metals which when combined,

produce a molten metal. This reaction, which is completed in seconds, takes place in a crucible. The

liquid metal flows from the crucible into a mold where it meets the ends of the conductors to be welded.

The temperature of the molten metal is sufficient to fuse the metal of the conductors, resulting in a

welded molecular bond. Exothermic welding alloys are available for aluminum, copper, and copper to

steel connections.

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BONDING TO THEEXTERNALGROUNDING(EARTHING) ELECTRODESYSTEM CHAPTER4: EXTERNALGROUNDING(EARTHING)

WARNING

To help prevent injury from molten metal or sparks and to reduce the risk of fire, followthe exothermic welding manufacturer's safety warnings and requirements.

Heavy clothing, work shoes or boots, gloves, and safety glasses shallbe worn when performingexothermic welding,

Exothermic welding shall notbe performed unless another person capable of rendering first aid is

present. A suitable fire extinguisher shallbe close by with an attendant during the process.

Observe the following prerequisites for exothermic welding:

Follow the manufacturer's recommendations.

Use the proper molds for the conductors being welded.

Use the proper weld material for the metals being welded.

Properly clean all metal parts prior to welding.

Properly dry all metal parts and molds prior to welding.

The exothermic welding process is shown in Figure 4-28.

CRUCIBLE

MOLD

MOLD CLAMPED

TO TOWER

FIGURE 4-28 EXOTHERMICWELDINGMOLD(LEFT) ANDPROCESS(RIGHT)

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STANDARDS ANDGUIDELINES FORCOMMUNICATIONSITES BONDING TO THEEXTERNALGROUNDING(EARTHING) ELECTRODESYSTEM

GROUND ROD

PIPE

EXOTHERMIC WELD

REBAR

EXOTHERMIC WELD

FIGURE 4-29 EXAMPLES OFCOMPLETEDEXOTHERMICWELDS

4.6.2.2 IRREVERSIBLEHIGHCOMPRESSIONFITTINGS

WARNING

Wear safety glasses, hard hat, and steel-toes shoes when working with high-compression fittings.

When using irreversible high-compression fittings, always use the compression tool recommended by

the manufacturer in accordance with the instructions provided by the manufacturer. Use fittings made of

the same material as the materials being bonded to avoid dissimilar metal reactions. See Figure 4-30forexamples of high-compression fittings.

Use fittings properly sized for the conductors being bonded.

Use fittings and compression tools rated at 13.3 tonnes (12 tons) of force.

Use only UL-listed connectors.

To ensure good contact, clean conductors using a wire brush before crimping.

Coat all crimped connections with a listed conductive antioxidant compound before crimping.

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MINIMUMSITEGROUNDING(EARTHING) REQUIREMENTS CHAPTER4: EXTERNALGROUNDING(EARTHING)

MECHANICAL HYDRAULIC BATTERY-POWERED

FIGURE 4-30 HIGH-COMPRESSIONCONNECTORS ANDTYPICALCRIMPINGTOOLS

4.7 MINIMUMSITEGROUNDING(EARTHING) REQUIREMENTS

This section provides the minimum grounding (earthing) requirements for installing a grounding

electrode system at a communications site and for bonding site equipment to the grounding electrode

system. Reasonable attempts shallbe made to achieve the grounding electrode system resistance design

goal, as defined in Grounding (Earthing) Electrode System Resistance Requirements on page 4-46

The requirements for installing a typical grounding electrode system are as follows:

Perform a soil resistivity test at the site as described in Appendix B, Soil Resistivity

Measurements

Calculate the resistance of a single ground rod as described in Interpreting Test Results on

page B-10.

Determine the resistance requirement of the grounding electrode system, based on the site type

(Light

chapter 4 motorola r56!09!01 05

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